In nuclear medical imaging techniques such as Positron Emission Tomography (PET) or Single Photon Emission Computed Tomography (SPECT) a subject ingests or is injected with a radiopharmaceutical. Typically the radiopharmaceutical comprises a radionuclide that is attached to a biologically or metabolically active molecule. A ring of photon detectors, or a photon detection ring, is placed around the subject to detect energetic photons emitted by the decay of the radio nuclide. This is then used to calculate the concentration of the radiopharmaceutical within the subject. When the radionuclide is attached to a chemical used in a metabolic process, the concentration of the radionuclide can be used to deduce physiological activity within particular tissues or organs. For example Fludeoxyglucose (18F), commonly abbreviated 18F-FDG, is a marker for the uptake of glucose by a tissue or organ. The spatial distribution of 18F-FDG can therefore be closely correlated with the metabolism of a subject.
PET and SPECT have the disadvantage that they do not provide details about the subjects internal anatomy and the response of the body to the radiopharmaceutical is relatively slow. Magnetic resonance imaging has been combined with, for example, PET to provide complementary information.
A large static magnetic field is used by Magnetic Resonance Imaging (MRI) scanners to align the nuclear spins of atoms as part of the procedure for producing images within the body of a patient. This large static magnetic field is referred to as the B0 field.
During an MRI scan, Radio Frequency (RF) pulses generated by a transmitter coil cause perturbations to the local magnetic field, and RF signals emitted by the nuclear spins are detected by a receiver coil. These RF signals can be used to deduce the concentration of certain atoms, such as hydrogen within the subject. This may be used to provide detailed information about the anatomy of the subject which can be combined with the information gained from a nuclear medical imaging technique. The chemical shifts of various resonances can also be determined to calculate the concentration of various chemical products within the subject using MRI also, using techniques referred to as functional Magnetic Resonance Imaging (fMRI). fMRI also provides data complementary to the nuclear medical imaging technique.
United Stated patent application US 2009/0299170 discloses combined MRI and PET unit. The magnet system of the MRI system is split by an azimuthal gap and the PET unit is disposed within the gap. The international application WO2008/122899 discloses a hybrid PET/MRI system. This known hybrid PET/MRI system has a split gradient coil assembly that has gradient coils connected by a stiff brace.